What little is known about the mechanism(s) by which the EC50 and PAA are determined derives from our studies of GR-regulated gene induction (reviewed in Simons Jr., 2003, TIPS, 24, 253-259;Simons Jr., 2006, Current Topics in Medicinal Chemistry, 6, 271-285;Simons Jr., 2008, Bioessays, 30, 744-756;Simons Jr., 2010, Current Opin. Pharmacology, 10, 613-619). However, the most commonly prescribed clinical use of glucocorticoids is for their capacity to repress gene induction, such as in the treatment of lymphomas by causing cell death and in the suppression of inflammatory responses. Furthermore, the mechanism of GR-regulated induction and repression is often different. Induction proceeds via GRs bound directly to DNA sequences called hormone response elements while repression often involves GRs indirectly bound to DNA through some other DNA-bound factor, such as AP-1 or NF-&#954;B. Finally, the EC50 of GR repression of gene expression is usually 10-fold lower than that for gene induction. Thus, at least some of the mechanistic details for GR-regulated induction and repression are different. Our studies of GR-regulated gene induction at physiological levels of steroid have documented that the Amax, EC50, and PAA for gene induction can be significantly altered simply by varying the concentration of a variety of transcription factors. As gene repression accounts for about half of all of the GR-mediated responses, it is clearly important to determine whether the same factors can similarly modulate the Amax, EC50, and PAA of GR-regulated repression. The approach of this study is to examine the details of gene repression for endogenous genes, rather than exogenous reporter genes. As described elsewhere (DK047039-04), we have recently prepared mouse embryo fibroblasts (MEFs) from wild type mice and mice in which the endogenous STAMP gene has been knocked out (KO mice). Microarray analyses have been performed on cells that were induced by glucocorticoid steroid for 8 hr. High quality data were obtained for almost 3,000 genes, for which the level of expression changed by &#8805;1.5 fold after Dex treatment. We are in the process of comparing the effects of STAMP-depletion on genes that are both induced and repressed by GR. At the same time, we are able to assess the role of STAMP on the PAA for induced and repressed genes because paired samples of MEF cells were treated at the same time with a saturating concentration of either agonist steroid or antiglucocorticoid. These studies are investigating whether our earlier conclusions regarding the modulation by cofactors of all three transcriptional parameters of GR-mediated gene induction (Amax, EC50, and PAA) can be extended to GR-regulated gene repression. These findings are contributing to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.